1. Field of the Invention
The present invention relates to a brush holder device, and in particular to a technique of preventing brush sound due to sliding of a brush in a brush holder made of resin and disposed opposite to a commutator, and, for example, to a brush holder device which can be effectively employed in a motor mounted on a vehicle.
2. Related Art Statement
As a conventional brush holder device used in a motor to be mounted on a vehicle, there is one in which, a brush is slidably inserted into a brush holder, and an action-side end portion of a torsion spring is engaged to an end surface of the brush on the side opposite to a commutator so that the torsion spring presses the brush against an outer peripheral surface of the commutator.
As described, for example, in Japanese Patent Laid-Open No. 8-154362, or Japanese Utility Model Laid-Open No. 4-137481, as this type of conventional brush holder device, there is one so constructed that an inclined surface is formed on an end surface (hereinafter, referred to as "back surface") of a brush on the side opposite to a commutator to prevent sound generated between the brush and an inside wall surface of a brush holder, accompanying sliding movement of the brush against the commutator. Namely, by engaging an action-side end portion of a torsion spring onto the back surface formed as the inclined surface of the brush, elastic force of the torsion spring is divided into component force along the normal of the commutator and component force pressing the brush against the inner surface of the brush holder's side wall, and thereby, sound generation due to sliding of the brush can be prevented.
Further, as shown in said 8-154362, the back surface of the brush is cut to provide an engaging groove with which the action-side end portion of the torsion spring is engaged, so as to stabilize behavior of the brush and to obtain good commutating action.
As for the above described brush holder device in which brush sound is prevented by the inclined surface, the present inventor has performed various tests on the component force for pressing the brush against the inner surface of the side wall of the brush holder. As a result, it have been found that, the inclination less than 25.degree. is insufficient in its effect, and, on the other hand, larger inclination improves the effect while it results in wide scattering of sliding friction between the brush and the inner surface of the brush holder's side wall. From these test results, it is desirable to set inclination of the inclined surface of the back surface of the brush at 30.degree.. The present inventor, however, has found that when inclination of the inclined surface is set at 30.degree., it becomes difficult to mold the brush by means of a molding die.
As the reason of difficulty, following phenomenon is considered. At the time of molding a brush by tamping in a molding die, particles of carbon, binder, and the like (hereinafter, referred to "brush particles") to form the brush show such a property that the particles align along a surface of a cavity of the molding die. In the case that an inclined surface is formed as a part of side surfaces of the cavity to form the inclined surface in the brush's back surface, alignment direction of the brush particles in the neighborhood of the inclined surface of the cavity conforms to that inclined surface, when the brush is molded. Accordingly, the alignment directions of the brush particles do not agree with a direction of tamping and become ununiform. When the alignment directions of the brush particles become ununiform, internal stress is generated, and thereby crack is produced in the brush. From the results of the above tests, a tendency has been found that this crack of the brush at the time of molding begins to be produced at an inclination of about 30.degree..
An object of the present invention is to provide a brush holder device which can ensure molding of a brush using a molding die, and, at the same time, can prevent generation of the brush sound.
On the other hand, to set the effective length of the brush long, it is necessary to connect a pigtail to the brush as near as possible to the back surface of the brush. When, however, an inclined surface is formed in the back surface of the brush, it is necessary to locate the pigtail away from the back surface by a length for avoiding the inclined surface. The larger the angle of inclination of the inclined surface is, the longer the length for avoiding the inclined surface is. Thus, in the brush having the inclined surface with the angle of inclination set at 30, the pigtail is connected to the brush much away from the back surface, and the effective length of the brush becomes shorter.
A second object of the present invention is to provide a brush holder device which can prevent decrease in the effective length of the brush.